Microsatellites or simple sequence repeats (SSRs) are co-dominant molecular markers. When we used fluorescent SSR markers to construct a linkage map for the female heterogametic silkworm (Bombyx mori, ZW), we found that some loci did not segregate in a Mendelian ratio of 1:1 in a backcross population. These loci segregated in a 3:1 ratio of single bands compared with double bands. Further examination of band patterns indicated that three types of SSR bands were present: two homozygotes and one heterozygote. In the beginning, we considered to discard these markers. By scoring male and female F1 individuals, we confirmed that these loci were located on the Z chromosome. Using the sex-linked visible mutation sch (K05) and its wild-type (C108), we constructed an F1 male backcross (BC1M) mapping population. The combination of sch backcross and SSR data enabled us to map the SSR markers to the Z chromosome. By adjusting input parameters based on these data, we were able to use Mapmaker software to construct a linkage map. This strategy takes advantage of co-dominant markers for positional cloning of genes on the Z chromosome. We localized sch to the Z chromosome relative to six SSR markers and one PCR marker, covering a total of 76·1 cM. The sch mutation is an important sex-linked visible mutation widely used in breeding of commercial silkworms (e.g. male silkworm selection rearing). Localization of the sch gene may prove helpful in cloning the gene and developing strains for marker-assisted selection in silkworm breeding.

Multiple sex-determining factors have been found in natural populations of the housefly, Musca domestica. Their distribution seems to follow a geographical cline. The ‘standard’ system, with a male-determining factor, M, located on the Y chromosome, prevails at higher latitudes and altitudes. At lower latitudes and altitudes M factors have also been found on any of the five autosomes. Such populations often also harbour a dominant autosomal factor, FD, which induces female development even in the presence of several M factors. Autosomal M factors were first observed some 50 years ago. It has been hypothesized that following their initial appearance, they are spreading northwards, replacing the standard XY system, but this has never been systematically investigated. To scrutinize this hypothesis, we here compare the current distribution of autosomal M factors in continental Europe, on a transect running from Germany to southern Italy, with the distribution reported 25 years ago. Additionally, we analysed the frequencies of the FD factor, which has not been done before for European populations. In contrast to earlier predictions, we do not find a clear change in the distribution of sex-determining factors: as 25 years ago, only the standard XY system is present in the north, while autosomal M factors and the FD factor are prevalent in Italy. We discuss possible causes for this apparently stable polymorphism.

The nature and extent of long-distance seed dispersal are currently poorly understood, largely due to the inherent difficulty in detecting such a phenomenon. Genetic methods provide one of the few general approaches that offer the potential to accurately address this issue. Phenotypic and allozymic approaches were applied to characterize inter-population seed dispersal of the sea rocket (Cakile maritima, Brassicaceae), a glabrous and succulent annual herb. Genetic variation was assessed on 360 individuals sampled from nine populations. Genetic diversity across populations was high, 37% of which was represented by Qst and 16% by Fst. When genetic distances were used to construct the UPGMA dendrogram, populations were clustered into three groups at the 90% similarity level. The pattern of clustering can be explained by examining the direction of sea currents around Tunisian coasts. We have shown in this study that C. maritima seeds can survive up to 4 months immersion in sea water and up to 1 year of floating in sea water; therefore, seed dispersal between populations is possible both in terms of seed survival and current patterns.

Statistical models for genetic evaluation often make use of Gaussian distributions. However, some new statistical developments allow the use of an asymmetric distribution for the residuals. Within this context, we analysed three different patterns for the residual term on a data set consisting of 63 208 litter-size records, belonging to 19 255 sows, with a pedigree including 27 911 individuals. The three different residual distributions were: (1) Gaussian distribution, (2) asymmetric Gaussian distribution and (3) asymmetric Gaussian distribution with a hierarchical scheme for the asymmetry parameter. The operational model always included order of parity and herd-year-season as systematic effects, and the permanent environmental and infinitesimal genetic effect of each sow as random effects. The most suitable model using the deviance information criterion (DIC) and posterior predictive checking was model 3. This implies systematic, additive genetic and permanent environmental control of both litter size and the asymmetry parameter of the residual distribution. The asymmetry parameter can be understood as a measure of sensitivity to negative (or positive) environmental influences on phenotypes. The posterior mean (standard deviation) of the additive genetic variance was 0·28 (0·06) for litter size and 0·07 (0·01) for the asymmetry parameter. The posterior mean (standard deviation) of the additive genetic correlation between litter size and the asymmetry parameter was 0·21 (0·07).

Wright's (1922) coalescence probabilistic method decomposes the average coancestry or the average inbreeding of the population of interest into contributions of ancestors who are directly responsible for coancestry. These ancestors are less numerous and younger than those given by Caballero & Toro's (2000) method. Then, Wright's method yields a more parsimonious representation of the origin of coancestry or inbreeding. Results of Wright's method, called proximal because ancestors are younger, can be obtained from the latter method using a simple matrix transformation. Detailed algorithms are presented. Both methods can be used for assessing the overall efficiency of a breeding scheme in balancing genetic gain and coancestry rate, yielding very similar results when generations are discrete.

Over recent years, selection methodologies have been developed to allow the maximization of genetic gain whilst constraining the rate of inbreeding. The desired rate of inbreeding is achieved by constraining the group coancestry using the numerator relationship matrix computed from pedigree. It is shown that when the method is applied to mixed inheritance models, where a QTL is segregating together with polygenes, the rate of inbreeding achieved in the region around a QTL is greater than the desired level. The constraint on group coancestry at specific positions around the QTL is achieved by using a relationship matrix computed from pedigree and genetic markers. However, the rate of inbreeding realized at the position of constraint is lower than that expected given the assumed relationship between group coancestry and the subsequent rate of inbreeding. The use of markers in the calculation of the relationship matrix allows the selection of candidates with very low or zero relationships because they are homozygous for alternative alleles, which results in a heterozygosity amongst their offspring higher than would be expected given their allele frequencies. A generation of random selection restored the expected relationship between group coancestry and inbreeding.

Departures of genotype frequencies from Hardy–Weinberg proportions (HWP) for a single autosomal locus due to viability selection in a random mating population have been studied only for the two-allele case. In this article, the analysis of deviations from HWP due to constant viability selection is extended to multiple alleles. The deviations for an autosomal locus with k alleles are measured by means of k fii fixation indices for homozygotes and k(k−1)/2 fij fixation indices for heterozygotes, and expressions are obtained for these indices (FIS statistics) under the multiallele viability model. Furthermore, expressions for fii and fij when the multiallele polymorphism is at stable equilibrium are also derived and it is demonstrated that the pattern of multiallele Hardy–Weinberg deviations at equilibrium is characterized by a global heterozygote excess and a deficiency of each of the homozygotes. This pattern may be useful for detecting whether a given multiallelic polymorphism is at stable equilibrium in the population due to viability selection. An analysis of Hardy–Weinberg deviations from published data for the three-allele polymorphism at the β-globin locus in human populations from West Africa is presented for illustration.